It's after the resignation of Edouard Philippe who has been in the post for three years.
Mr Castex, 55, is relatively unknown in France, although he is a senior civil servant.
The new Prime Minister has been a key player in the government's response to the Covid-19 pandemic.
Drone, missile attack on Kyiv injures 11, damages buildings
Russian forces launched a massive drone and missile attack on Kyiv early on Friday, striking residential buildings and triggering explosions and fires in districts throughout the capital, officials said.
France commemorates victims of deadly Paris attacks 10 years on
France paid an emotional tribute on Thursday to 130 people killed 10 years ago during a rampage by IS gunmen and suicide bombers targeting cafes, restaurants and the Bataclan concert hall.
Afghan nationals carried out two bombings in Pakistan, says minister
Afghan nationals carried out two bombings in Pakistan this week, including one in the capital Islamabad, Pakistan's Interior Minister Mohsin Naqvi told the parliament on Thursday.
Trump signs deal to end longest US government shutdown in history
President Donald Trump has signed a legislation on Wednesday ending the longest government shutdown in US history, bringing federal workers idled by the 43-day shutdown back to their jobs starting as early as Thursday.
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